The present invention relates to an electrophotographic photoconductor comprising an electroconductive support and a photoconductive layer comprising a particular bisazo pigment as a charge generating material which generates charge carriers when exposed to light.
Conventionally, a variety of inorganic and organic electrophotographic photoconductors are known. As inorganic electrophotographic photoconductors, there are known, for instance, a selenium photoconductor, a selenium-alloy photoconductor, and a zinc oxide photoconductor which is prepared by sensitizing zinc oxide with a sensitizer pigment and dispersing the same in a binder resin. Furthermore, as a representative example of organic electrophotographic photoconductors, an electrophotographic photoconductor comprising a charge transporting complex of 2,4,7-trinitro-9-fluorenone and poly-N-vinylcarbazole is known.
However, while these electrophotographic photoconductors have many advantages over other conventional electrophotographic photoconductors, they have several shortcomings from the viewpoint of practical use.
For instance, a selenium photoconductor, which is widely used at present, has the shortcomings that its manufacturing conditions are difficult and, accordingly, its production cost is high, and it is difficult to work it into the form of a belt due to its poor flexibility. Furthermore, it is so vulnerable to heat and mechanical shocks that it must be handled with the utmost care.
In contrast to this, the zinc oxide photoconductor is inexpensive since it can be produced more easily than the selenium photoconductor. Specifically, it can be produced by simply coating inexpensive zinc oxide particles on a support. However, it is poor in photosensitivity, surface smoothness, hardness, tensile strength and wear resistance. Therefore, it is not suitable for a photoconducot for use in plain paper copiers in which the photoconductor is used in quick repetition.
The photoconductor employing the aforementioned complex of 2,4,7-trinitro-9-fluorenone and poly-N-vinylcarbazole is also poor in photosensitivity and therefore not suitable for practical use, particularly for a high speed copying machine.
Recently, extensive studies have been done on the electrophotographic photoconductors in order to eliminate the above-mentioned shortcomings of the conventional photoconductors. In particular, attention has been focused on a multi-layered type organic electrophotographic photoconductor, each comprising an electroconductive support, a charge generation layer comprising an organic pigment formed on the electroconductive support, and a charge transport layer comprising a charge transporting material formed on the charge generation layer, which are for use in plain paper copiers, since such multi-layered type organic photoconductors have high photosensitivity and stable charging properties when compared with the conventional organic photoconductors. As a matter of fact, several types of multi-layered type organic electrophotographic photoconductors are being successfully used in practice. Examples of the multi-layered type organic electrophotographic photoconductors are as follows:
(1) A multi-layered type electrophotographic photoconductor whose charge generation layer is prepared by vacuum evaporation of a perylene derivative and whose charge transport layer comprises an oxadiazole derivative, disclosed in U.S. Pat. No. 3,871,882.
(2) A multi-layered type electrophotographic photoconductor whose charge generation layer comprises Chlorodiane Blue which is dispersed in an organic amine solution and coated on an electroconductive support and whose charge transport layer comprises a hydrazone derivative, disclosed in Japanese Patent Publication No. 55-42380.
(3) A multi-layered type electrophotographic photoconductor whose charge generation layer comprises a distyrylbenzene type bisazo pigment dispersed in an organic solvent and coated on an electroconductive support, and whose charge transport layer comprises a hydrazone compound, disclosed in Japanese Laid-Open Patent Application No. 55-84943.
(4) A multi-layered type electrophotographic photoconductor whose charge generation layer comprises a bisazo compound represented by the following formula, disclosed in U.S. Pat. No. 4,486,519: ##STR2##
(5) A multi-layered type electrophotographic photoconductor whose charge generation layer comprises a bisazo compound represented by the following formula, disclosed in Japanese Laid-Open Patent Application No. 62-273545: ##STR3##
As previously mentioned, these multi-layered type electrophotographic photoconductors have many advantages over other electrophotographic photoconductors, but at the same time, they have various shortcomings.
Specifically, the electrophotographic photoconductor (1) employing a perylene derivative and an oxadiazole derivative presents no problem for use in an ordinary electrophotographic copying machine, but its photosensitivity is insufficient for use in a high-speed electrophotographic copying machine. Furthermore, since the perylene derivative, which is a charge generating material and has the function of controlling the spectral sensitivity of the photoconductor, does not necessarily have spectral absorbance in the entire visible region, this photoconductor is not suitable for use in color copiers.
The electrophotographic photoconductor (2) employing Chlorodiane Blue and a hydrazone compound exhibits comparatively good photosensitivity. However, when preparing this photoconductor, an organic amine, for example, ethylene diamine, which is difficult to handle, is necessary as a coating solvent for forming the charge generation layer.
The electrophotographic photoconductor (3) employing a distyryl benzene type bisazo compound and a hydrazone compound have an advantage over other conventional electrophotographic photoconductors in that the charge generation layer can be prepared easily by coating a dispersion of the bisazo pigment on an electroconductive support. However, the photosensitivity of the photoconductor is too low to use as a photoconductor for a high-speed electrophotographic copying machine.
Furthermore, recently there is a demand for a photoconductor for use in laser printers, in particular, for a highly sensitive photoconductor having a photosensitivity in a semiconductor laser wavelength region.
Even the electrophotographic photoconductor (4) and (5) employing the previously mentioned bisazo compound(s) show insufficient absorbance near 780 nm of a semiconductor laser wavelength region.
Any of the above-mentioned photoconductors has too low a photosensitivity to use in practice for semiconductor laser.